1. Field of the Invention
The present invention relates to a focusing error detecting device in an optical head assembly for reading or writing information from or to an information recording medium such as an optical disc. Particularly, it is concerned with a focusing error detector in an optical head using optical waveguides.
2. Description of the Prior Art
FIGS. 1(a) and (b) illustrate a construction of a conventional optical pickup using a optical waveguide disclosed in Japanese Laid-Open Application No. 130448/83. In these figures, the reference numeral 1 denotes a light source, e.g. a semiconductor laser, and the numeral 2 denotes a optical waveguide layer formed by a dielectric thin film on a substrate 3. Guided beam emitted from the light source 1 and propagated through the optical waveguide layer 2 is taken out into space and converged to a focused spot 6 by means of a first diffraction grating 5. The numeral 7 denotes an information recording medium such as an optical disc; numeral 8 denotes a second diffraction grating provided on the back of the substrate 3 for conducting a reflected beam from an information recording surface 9 of the information recording medium 7 to a photo detector 10; and numeral 11 denotes a converged spherical wave.
The first diffraction grating 5 is for converting a guided beam 4 propagated through the thin-film optical waveguide layer 2 into the converged spherical wave 11 at the focused spot 6. The pattern of the diffraction grating 5 is determined from a difference in phase between the converged spherical wave 11 on the thin-film waveguide layer 2 and the guided beam 4 on the same layer. The diffraction grating 5 is named a focusing grating coupler (FGC) in view of its function and is reported in Heitmann et al., "Calculation and Experimental Verification of Two-Dimensional Focusing Grating Coupler", IEEE Journal of Quantum Electronics, QE-17, pp. 1257-1263 (1981), and also in Matsuoka, Suhara, Nishihara and Koyama, "Focusing Grating Coupler by Electron Beam Drawing", Research Meeting Report MW83-88, Institute of Electronics and Communication Engineers of Japan, pp. 47-54 (1983). Part of the light reflected from the information recording medium 9 passes through the thin-film waveguide layer 2 and the substrate 3 and reaches the back of the substrate. The second diffraction grating 8 functions as both a cylindrical lens and a converging lens, and it creates an astigmatism on the transmitted wavefront. As to this diffraction grating lens, the inventors in the Japanese laid-open application No. 130448/83 make report in Applied Physics Society Biannual Meeting - Draft Lectures, 26p-s-5, p. 170 (autumn 1983). In the figures, a reflected beam 12 having astigmatism is conducted to the photo detector 10, whereby a focusing error and a tracking error can be detected by an astigmatism method and a push-pull method which utilizes bisection of beam.
Thus, in the optical head assembly shown in FIG. 1, the processing for the reflected beam for focusing error and tracking error detection is performed by the photo detector 10 which is disposed spatially away from the optical waveguide layer 2. Consequently, it is necessary to make a triaxial adjustment of the photo detector 10, which makes it impossible to attain the reduction in size of the optical head.